4500+@75mph? What rear setup do you have...5.38 gears and little lawnmower tires? Any engine that has to run that many R's is bound to suck the gas.... A long stroke slant especially is happier at lower revs, so I am left wondering about the accuracy of your tach... is it an aftermarket one with the little switch in back on 4 cyl? Then it would read twice your actual RPM's..... But as a front end alignment guy that has swapped slants for 8's before, my 79 D100 busted a front spring about 15K after the swap, I have changed many sets of springs in my job, and I remember Moog only showing 2, possibly 3 options on springs for Dodge trucks/vans of that vintage, regardless of engine or 1/2,3/4, or 1 ton rating... You gotta remember when you load down the back, where does the front end go? UP! Do not go with the Heavy Duty listing in this instance unless you run something like a big block dually with a plow or a tow truck with a huge front push bumper, the standard will work out nicely for you.
Catch ya later,Don